Acoustic flame detectors for steam generators

ABSTRACT

In a steam generator having a plurality of burners fed by a common windbox and heating a common firebox, a directional electroacoustical sensor for each burner is disposed in the passage between the windbox and the firebox. The sensor is responsive to a relatively narrow band width of sound and distinguishes between a flame and a no-flame condition in the individual burner.

United States Patent [191 Whitman y ACOUSTIC FLAME DETECTORS FOR STEAM GENERATORS [75 Inventor: Nelson Whitman, Lincoln, Mass.

[73] Assignee: General Electric Company,

Schenectady, NY.

[22] Filed: Jan. 29, 1973 [2]] Appl. No.: 327,429'

52 us. or. 431/75 [51] int. Cl. F23m 5/16 [58] Field of Search 431/75,.79, 167, 178, 180

[56] I References Cited UNITED STATES PATENTS I 3,216,477 l1/1 965 Devine 431/79 3,635,018 1/1972 DeCorso eta] 1. 431/75 X I. Y O u [11], 3,811,816 [451 .May 21, 1974 3,280,882 10/1966 Hemker... 431/79 2,767,783 10/1956 Rowell et a1. 431/12 Mitchell 57 ABSTRACT in a steam generator having a plurality of burners fed by a common windbox and heating a common firebox, a directional electroacoustical sensor for each burner is disposed in the passage between the windbox and the firebox. The sensor is responsive to a relatively narrow band width of sound and distinguishes between a flame and a no-flame condition in the individual burner.

4 Claims, 6 Drawing Figures ACOUSTIC FLAME DETECTORS FOR STEAM GENERATORS BACKGROUND OF THE INVENTION In the operation of marine and land-based steam generating plants, it is often-necessary to operate at very low as well as very high fuel rates. To operate at low fuel rates, it has been found preferable to reduce the fuel burned in all of the burners rather than to'shut off one or more of the burners. Atlow fuel flow, a momentary interruption in fuel flow to one of the burners may create a danger of explosion, since the flame may not be re-established until a large amount of fuel has been fed into the firebox and this fuel reaches one of the flames in a burner at a different location.

It is well known that an experienced ear can detect the difference in sound between a burner which is ignited and one which is not ignited. However, the current tendency toward unattendedfirerooms, particularly aboard ship,- makes it desirable to provide an automatic flame detector. Acoustic flame detectors have been suggested in the prior art, both for boilers and gas turbine combustion systems. U;S. Pat. No. 2,767,783 to Rowell et al. teaches the method of controlling the supply of fuel to a single burner by locating a detector in the firebox door and measuring the, presence or nonpresence of 'a flame in a preferred sound frequency range of about 20 to 150 hertz (cycles per second). De Corsoet al. US. Pat. No. 3,635,018 teaches location of a sensor directed toward .the interior of a gas turbine combustion liner and using filters to .reject unwanted noise in the 6,000 16,000 hertz band. In the Rowell patent, the detector .listens to sound inside the firebox which is fed by a single burner. In the De Corso patent, it is suggested that a sensor can be employed for each of several combustion chambers, where the combustion chambers arenot interconnected'by cross-fire tubes.

Accordingly, one object of the present invention is to provide'an improved acousticflame detector system designed to detect the failure of one of a plurality of burners, particularly at low fuel flow and to shut off the fuel to the extinguished burner.-

Another object of the invention is to provide an improved flame detector suitable for a steam generator having a plurality of closely disposed burners.

Still another object of the invention is to provide an improved method for detecting flame failure in an individual burner and shutting down the burner to prevent explosion.

f DRAWING The subject matter. which is regarded as the invention is particularly pointedout and distinctlyclaimed in the concluding portion of the specification. The invention, however, both as to organization and method of practice, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawing, in which:

FIG. 1 is a front elevation view of the lower part of a typical marine boiler.

FIG. 2 is a horizontal cross section of a typical fuel burner in such a boiler,

FIG. 3 is a schematic block diagram of the probe amplifier and burner shutoff system,

FIGS. 4a and 4b are graphs showing the decibel and the change in decibel level for one type of burner, and

FIG. 5 is a graph showing the change in decibel level for a different type of burner.

SUMMARY OF THE INVENTION Briefly stated, the invention is practiced by locating a directional electroacoustical sensor in the air supply passage of each individual burner between a common windbox and a common firebox. A relatively narrow frequency band of sound selected to give the greatest decibel difference between flame and no-flame conditions is monitored for each burner. Sound level below a selected threshold in a particular burner shuts off fuel to that burner.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 shows the lower front half of a marine boiler 1 having four individual burners designated 2, 3, 4 and 5. Each burner has a removable fuel manifold assembly 6 and an air register handle 7 to control the flow of air betweenthe windbox and the firebox. In accordance with the present invention, each burner also includes an acoustic probe 8, with shielded electrical leads 8a, arranged in a manner to be described in more .detail.

Referring to FIG. 2 of the drawing, a typical marine burner assembly is shown. Although such burners may take many different forms according to the particular manufacturer, the burner shown is a Racer type manufactured by Babcock and Wilcox and will be used to describe theprinciple of the invention. The manifold assembly 6, air register handle 7 and probe 8 are preferably mounted in a removable insulated cover plate 9 which is contained in the outer furnace wall 10. Spaced from the outer wall 10 is the inner or insulated firebox wallll provided with an opening 12 for each burner leading to a common fireboxv 13. The space between outer wall l0and inner wall 11 is known in the art as the windbox. 14 and this is supplied with combustion air by a, suitable fan. A controllable flow of air from windbox 14 to firebox 13 is selected by the setting of rotatable air register vanes 15 in accordance with the position of the airregister handle 7 by means of an interconnecting gear mechanism 16.

Liquid fuel and atomizing steam are fed to the spray nozzle 17 from the manifold 6. A rotatable lever 18 allows the withdrawal of the supply housing 19 and the connected nozzle 17 for cleaning. Additional means for controlling flame pattern, such as a flame shield 20 and swirl vanes 21, vary with the type of burner and are not material 'to the present invention. However, all of the burners to which the present invention applies have a construction which allows the selective admission of air from a common windbox through an individual air admission passage along with the fuel into a common firebox.

In accordance with the present invention, an acoustic high temperature probe 8 is mounted in cover plate 9 by means of a bracket 22 and a suitable aperture 23. The probe comprises an electroacoustical microphone in a hollow tube which extends through the windbox 14 'and terminates in a space 24 which is behind the burning fuel emitted from nozzle 17, but within the individual air supply passage controlled by register vanes portion 25 may be provided in one vane to allow it to rotate. Details for insertion of the probe will necessarily vary from one type of manufacturer to the other depending upon the air controlling means into chamber The probe is selected to be resistant to the temperatures expected to be encountered near the burner and is also selected to be highly directional along the probe axis. If a more sensitive probe is desired, water or air cooling may be employed. The probe is oriented so that it is directionally most sensitive toward the flame itself. The probe should also be mounted and oriented to attenuate any extraneous unwanted noises which would tend to lower the overall sensitivity of the system.

FIG. 3 of the drawing shows a block diagram for two burners. Elements having comparable functions in FIGS. 2 and 3 have the same reference numbers. Probes 8 for a pair of burner openings 12 are shown to be located in the individual air passages 12 between the common windbox 14 and common firebox 13. Each probe is shown with a separate amplifier 31 connected to an acoustic .band filter 32 which is adjustable to pass a selected bandwidth of sound. Output from the filter passes to a threshold trigger network 33 which provides a signal when the sound level falls below a selectable value. The latter signal is adapted to actuate a burner fuel shutoff valve 34.

In the foregoing arrangement, the electronic devices are conventional. Amplifiers and filters for this service are available from Bolt, Beranek & Newman and others. The trigger circuit may be a conventional meter relay.

The selection of the proper bandwidth and frequency is highly empirical. It has been found to differ not only with the type of burner but also the type of fuel being burned. However the preferable frequency range within which the bandwidth is located is preferably between 200 hertz and 5,000 hertz. I have discovered that the proper criteria for selection of frequency and bandwidth for the acoustic probe is not the total sound level at a particular frequency, but rather is the difference in sound level between a burner with flame and a burner with no flame at a particular frequency. FIG. 4a illustrates the sound level in one-third octave bands as measured in decibels versus frequency in hertz for an industrial furnace burning No. 6 fuel using a burner supplied by Foster-Wheeler C0. Curve 40 is the absolute sound level in decibels at various frequencies when the air atomizing steam and fuel are flowing into the burner but no flame is taking place. Curve 41 illustrates sound level when the burner is ignited at a low rate of fuel flow and curve 42 when the burner is ignited at maximum fuel flow.

FIG. 4b corresponds to FIG. 4a, but shows the difference in decibels between the absolute sound level at no flame and the absolute sound level with flame. Thus curve 43 is a plot of the difference between curves 40 and 41 of FIG. 4a. Similarly curve 44 is a plot of the difference between curve 40 and 42 of FIG. 4a.

In accordance with the present invention, a relatively narrow frequency band is selected as indicated on FIG. 4b by dashed lines 45, 46 encompassing a one-third octave band centered on a frequency of 500 hertz, where the difference in decibels between the flame and no flame shows a suitable value. One-third octave bandwidths have been used in this example because of the ready availability of filters to pass this width band. Al-

though a band of frequencies around hertz might also might have been selected, this is avoided because of its nearness to a 60 hertz frequency which is associated with extraneous noise sources.

A suitable threshold level, I08 decibels in the example shown, is selected as indicated at 47 on graph 4a. The threshold trigger circuit 33 is arranged to actuate the burner fuel shutoff relay panel 34 when the signal falls below this level.

To further illustrate the method of the present invention and the fact that the proper'frequency band depends upon the particular burner and type of fuel, FIG.

5 shows a graph similar to FIG. 4!), but for a John Zink Co. experimental burner burning No. 6 fuel oil. Although a one-third octave band might have been selected centered at 500 hertz instead a one-third octave band emcompassed by dashed lines 48, 49 centered about a frequency of 2,500 hertz is selected. Curve 50 is the difference in decibels between flame and no flame for a maximum flow of fuel, while curve 51 is the corresponding decibel difference curve for a minimum flow of fuel. A suitable signal difference for both of these fuel flow conditions is obtained at 2,500 hertz as indcated by the substantial coincidence of the curves 50, 51. Plotting of curves similar to FIG. 4b and FIG. 5 is necessary in order to obtain the optimum selection of a frequency band and a frequency for a particular type of burner and a particular fuel.

OPERATION The invention operates as follows. When all four burners are operating properly, the sound level for each burner is above the threshold level within the bandwidth of sound being amplified and passed by the filter. Interruption of flame at one of the burners causes the sound level to drop below the threshold and to actuate the relay to shut off fuel to the affected burner.

The remaining burners continue to function as long as common windbox and firebox, the probe location as' indicated enables the probe to distinguish the sound due to flame or no flame at an individual burner and to thereby control the fuel flow.

While there has been described what is considered at present to be the preferred embodiment of the invention, other modifications will become apparent to those skilled in the art, and it is desired to secure in the appended claims all such modifications which fall within the true spirit and scope of the invention.

What is claimed is:

1. In a steam generator having a plurality of burners, each supplied with air from a common windbox and each ejecting a combustible mixture into a common firebox, each of said burnersincluding an air duct with an adjustable air register interposed between said windbox and said firebox and having a fuel nozzle disposed in said air duct, an acoustic detector system comprising:

a plurality of electroacoustical probes, each having a directional characteristic. and disposed with one 2. The combination according to claim I, wherein said probe comprises a hollow tube extending across said windbox and terminating between said air register and said nozzle, said tube having an electroacoustical microphone disposed therein.

3. The combination according to claim 1, wherein said selected bandwidth is on the order of one third of a frequency octave as measured in hertz.

4. The combination according to claim 1, wherein said selected bandwidth is selected to lie within a band of 200 hertz to 5,000 hertz.

Disclaimer and Dedication 3,811,816.-Nelson Whitman, Lincoln, Mass. ACOUSTIC FLAME DETEC- TORS FOR STEAM GENERATORS. Patent dated May 21, 1974. Disclaimer and dedication filed Dec. 1, 1980, by the assignee, General E Zeatric 0' ompany. Hereby disclaims and dedicates to the Public all claims, 1 through 4 of said patent.

[Oficial Gazette Febmary 10, 1981.] 

1. In a steam generator having a plurality of burners, each supplied with air from a common windbox and each ejecting a combustible mixture into a common firebox, each of said burners including an air duct with an adjustable air register interposed between said windbox and said firebox and having a fuel nozzle disposed in said air duct, an acoustic detector system comprising: a plurality of electroacoustical probes, each having a directional characteristic and disposed with one end terminating within a respective air duct on the upstream side of said fuel nozzle therein and directed toward the firebox. means for amplifying a bandwidth of sound sensed by said probe, said bandwidth being selected to provide a substantial decibel difference between a flame and a no-flame condition for each of said burners, said amplifying means providing a signal indicative of sound level within said bandwidth, and means for interrupting fuel to an individual burner when said signal associated with said individual burner falls below a selected value.
 2. The combination according to claim 1, wherein said probe comprises a hollow tube extending across said windbox and terminating between said air register and said nozzle, said tube having an electroacoustical microphone disposed therein.
 3. The combination according to claim 1, wherein said selected bandwidth is on the order of one third of a frequency octave as measured in hertz.
 4. The combination according to claim 1, wherein said selected bandwidth is selected to lie within a band of 200 hertz to 5,000 hertz. 